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JSTOR helps people discover, use, and build upon a wide range of content through a powerful research and teaching platform, and preserves this content for future generations. JSTOR is part of ITHAKA, a not-for-profit organization that also includes Ithaka S+R and Portico. For more information about JSTOR, please contact support@jstor.org. STOR Philosophical Transactions Please note: Due to an error in the print volume, the page numbering in this article may contain either page numbering skips, or page numbering repetitions, or both. However, the article content is presented in its entirety and in correct reading order. Please click on "Next Page" (at the top of the screen) to begin viewing the article. (* $084 ) M>. Ifaac Newtons Anfiver tofome Confederations upon his Dot Brine of Light and Colors 5 nbich DeUrim was printed in Numb.. 80'. ofthejefraUs. Sl}\, 1 have already told you, that at the perufal of the con- iider3tiobs, you fent me, on my' Letter concerning Refrac- tions and Colors^ \ found nothing, that, as I conceived,mighc not without difficulty be anlwer'uV And though I find the Confederer fomewhat more concerted for an Hypotheps, thau I expected } yet I doubt not, but we have one common defiga ; I mean, a fincere endeavour af cer knowledge, without valuing uncertain (peculations for their fubtleties, or defpifing cer- tainties for their plainnefs : And on confidence of this it is, that I make this return to his difcourfe.* Thefirft thing that offers it fe \i mdhltfkUr.phtau im;k is left agreeable tome, and I begm iecaufe in the loly of tbh Snf-wer with it becailfe it IS fb. The COnll- atiHkmfuitbihcKtfpmhM- d e - s p l e afed to reprehend me larf, whetttu the Anfwerer was r . . * r , , , r , Z, ' tenor*' 4. *or laying ahde the thoughts of im- tipth If he had obliged me by a private Letter on this occafion, I would Have acquainted him with my fuccefles on the Tryals I have made of that kind, which 1 fhall now fay have been lefs than 1 lometimes expected, and perhaps than he at prcfent hopes for. But finceheispleafedto take it for granted, that I have let this fubject pais without due examination, I mall refer him * » • j ■ « u r , r to my former Letter, * by which * Pnnttd m Numb. go. tf ibeft . J - « .,, J . mfc. that conjecture will appear to be un-grounded. For, what I faid there, was inrefpeft of Telefcopes of the ordinary conftrudN on, Signifying, that their improvement is not to be expected from the welhfiguringoi Glades, as Opticians have imagin'd j but I defpaired not of their improvement by other conftru- dtions 3 which made me cautious to infert nothing that might intimate the contrary. For, although fucceffive refractions that are all made the fame way, do necefTarily more and more augment theerrors of the firft refraction § yet it feem'd not impofliblefor contrary refractions fo to correct: each others inequalities, as to make their difference regular 5 and, if that could r $085 > could be conveniently efFe&ed, there would be no further dif». ficulty. Now to this end Iexamin'd, what may be done not only by Gtaffisalont, but more efpecially by a Complication of divers i'ucceffiv e Me^/aw/, as by two ormoreGlafies or Cry- ftals with Water or fome other fluid between them; all which together may perform the office of one Glafs t efpecially of the Objed-glafs, on whofecouftrudtion the perfection of tke in- ftrument chiefly depends* But what the refults in Theory or by Tryals have been, I may poilibly find a more proper occa- fion to declare. To the AfTertion, that tvays are lefs true reflected to a point by a Concave, than refracted by a Convex, I cannot aflfent 3 nor do I understand, that the jW* of the latter is lefs a line than that of the former. The truth of the contrary you will rather perceive by this following Table, computed for fuch a Reflect ting Concave, and Refracting convex, on fuppoficion that they have equal Apertures, and colled: parallel rays at an equal di' ftance from their vertex j which diftance being divided into 15000 parts, the Diameter of the Concave Sphere will be 60000 of thofe parts, and of the Convex, 10000 5 fuppofing the Sines of Incidence and fvefra&ion to be, in round num- bers, as 2 to 3* And this Table ihews, how much the exterior rays, at feveral Apertures, fall fliort of their principal focttt. The farts of the Axis intercepted The Diameter between the vertex *nd the rays, of the Aperture 2000 4000 6000 8000 1 0000 JRefle&eA. 1499 if 14966 14924 14865 14787 Refrstted. I4865" 14449 13699 I247S 9472 The Error iy He flexion Refrattion ___ iTjT 3 3 5S* • 76 1301 . 135 25 $ : 213 5528 . By this you may perceive, that the Errors of the Refracting eonve x are fo far from being lefs, that they are more than fix- teen times greater than the like errors ot the RjflcctingConcpve, efpecially in great Apertures 5 and that without refped: to the Heterogeneous couftitution of light. So that, however the contrary fuppofition might make the Author ofthefe Animad* verfionsreje<5t Reflections as ufelefs for the promoting of Op* K k k k k 2 tiques ( %o%6 ) tiques ; yet I muft for this as well as other confideratio lis pre fer them intheTheory before Refractions. Whether the Parabola be more difficult to defcribe than the Hyperbola or Ellipjis, may be a Qu\zxx£\b\c confequencc of the Doctrine, and not a funda- mental Suppofition, nor fo much as any part of it • which was wholly comprehended >n the precedent Propofitions r Audi lbmewhat wonder, how the Objector could imagine, that,when I had aiTerted the Theory with the greateft rigour, 1 mould be fo forgetful as afterwards to affert the fundamental fiippo^ fition it felf with no more than a perhaps. Had I intended any inch Hypotbefis, 1 mould fomewhere have explained it. But I knew, that the Properties, which 1 declar'd of £/g/;/,were in fome ( 5o8 7 ) fome meafure capable of being explicated not only by that, but by many other Mechanical Hypotbejes. And therefore I chofe to decline them all, and to fpeak of Light in general terms, considering itabftra&Iy, as foaaething or other propa, gated every way in ftreight lines from luminous bodies, with, out determining, what that Thing is; whether a confufed Mixture of difform qualities, or Modes of bodies, or of, Bod ^bies theuijelves;, or of any Virtues, Powers, or Beings what- fbever* And for the lame reafou I chofe to fpeak of Colours according, to the information of our Senfes, as if they were Qualities of Light without us. Whereas by that Hypotbijis I muft have coofidered them rather as Modes of Seniation, ex» cited in the mind by various motions, figures, or fizes of the cprpulclesofL'ght, making various Mechanical impreffions onthe Org-in of Senfe$ as I exprefled it in that place, wlierc I ipake of the Corporeity of Light. Butfuppofihg I had propounded that Hypothc fit, I under- ftand not, why the Obje&of Ihould fo much endeavour to op- pofeir. For certainly it has a much greater affinity with his own Hypotbejisy than he feems to be aware of\ the Vibrations of the JEther being as ufeful andneceflary in this t as in bis .For i afliiming the Rays of Light to be fmall bodies, emitted evejy way from Shining fubftances, thofe, when they impinge on any Refracting or Refle&ing fuperficieSj. muft as neceiTarily excite Vibrations in the tether t as Stones do in water when thrown into it. And fuppofingthefe Vibrations to be of (c- veral depths or thickneffes^ accordingly as they are excited by the laid, cor puicular rays of various fizes and velocities^ what ufe they will be for explicating the manner of Reflection and Refraction, the production of Heat by the Sun-beams, the &■ miffion of Light from burning putrifying, or other fubftances, whofe parts are vehemently agitated, the Vh&nwuna of thin tranf parent Plates and Bubles s and of all Natural bedier, the Manner of Vifion, and the Difference of Colors, as alio then tjarraony and Difcord j / mail leave to their confid:ration , who may think it worth their endeavpr to apply this Hypnthfu to the fa lut i on oiphtnomen** C 508S ) la the fecotwipkce, I cold you,that the Objectors Hypothecs, ,....„,„ , ,. , a«o the fundamental part of it, is tffi^&gSfrSl not againft me. That-fundamental ■jut*tit*tf that and all etbtrMecbM- Suppofition is; That the parts of £oetri%r htJil " mf " m * bU " mj bodies, when briskly agitated, do ex- cite Vibrations in the ALther t which are propagated every way fromtbofe bodies in fir eight lines, and caufe a Senjation of Light by beating and dafhinp^ againfi the bottom of the Eye 9 (ometbmg after the manner that Vibratims in the Air caufe a Senjation of Sound by be/ating againfi the Organs of Hearing. Now, the moft free and natural Application of this Hypothecs to the Solution oipbanomena I take to be this : That the agitated parts of bodies, according to their feveral fizesj figures, and mo* tiom, do excite Vibrations in the atberot' various depths or kigncffeSjWhich being protnifcuoufly propagated through that Medium to our Eyes.effect in us a Senfation of Light ol a White colour ; but if by any means thofe of unequal bignefles be fe-- parated from one another, the largeft beget a Senfation of a Red colour^ the leaft or fhorteft, ot a deep Violet, and the in- termedial ones, of iotermediat colors; much after the man- ner that bodies , according to their feveral fizes, fhapes, and motions, excite vibrations in the Air of various bignefTes , Which, according to thofe bignefTes, make feveral Tones in Sound: That the largeft Vibrations are belt able to over- come the refiftance of a Refracting fuperficies, and f© break through it with leaft Refraction 5 whence the Vibrations of feveral bignefTes, that is, the Rays of feveral Colors, which are blended together in Light, muft be parted from one ano- ther by Refraction, and fo caufe the fhxnomena of Pr/fmes and other refracting fubftances ; And that it depends on the thick - nefsofa thin tranfparent Plate or Buble, whether a Vibration {hall be reflected at its further fuperficies,or tranfmitted; fo that, according to the number of vibrations, interceding the two fuperficies.they may be reflected or tranfmitted for many fuc- ceffive thickneffes. And fince the Vibrations which make Blew and Violet % are fuppofed fhorter than thofe which make RedandTellow, they muft be reflected at a lefs thickuefs of the Plate ; Which is fufficient to explicate all the ordinary phtenei mma of thofe Plates or Bubles, and alfo of all natural bodies, whofe ( 5**9 ) whofe parts are [like fa many fragments of fuch Plates, Thefe feem to be the moft pfatD, genuine-and neceffary conditions of this Hypt>theJ!j:Au6 they agree fojuilly with my Theory, that if the Animadverfvr think fit to apply them, he need not, on that account, , . apprehend a divorce from ir. But yet how he will defend it from other difficulties, I know nor. for, to me, the Fnndaia^eotal Sappofition it ielf feems impof- fible ; namely ,Tbat the Waves or Vibrations o£ any FluidjCan, like the Rays of Light, be propagated in S freight lines,without a continual andvery extravagant fpreading and bending every way into the quiefcent Medium, where they are terminated by it. 1 miftake, if there be not both Experiment and De- monftration to the contrary* And as to the other two or three Hypothecs, which he mentions, I had rather believe them fub* ject to the like difficulties,than fufpc& theJmmadverfjp ftiould leledfc the worft for his own. What I have faid of this, may be eatlly applied to all other Mechanical Hypothsfes, in which Light is fuppofed to be caufed by any Preffion or Motion whatfoever, exeked in the tether by= the agitated parts of Luminous bodies.For,it feems impoffible, that any of thofe Motions or Prcffions can be propagated in streigbt lines without the like fpreading every way into the fhadow'd Medium, on which they border. . But yet, if any man can think it poffible, he muft at leaftallow,that thofe Mo* tions or Endeavors to motion, caufed in the eo~ *?f*Jf ?£?££* S^'f ? , ., ,/J. i/-, ,j •»'» and mtr hrnitttien te bis Hypo- net by his Hypuhefis: And if I could ,t,^, comply with him in that point, there r 5090 1* there would be little or no difference betweea Us. For he grants, that without any refpect to a different Incidence of rays there are different Refractions * but he would have it ex- plicated, not by the different Refrangibility of feveral Rays, but by the' Splitting and Rarefying of aethereal pulfes. He grants my third, fourth and fixth Propositions ; the fenfe of which is, That Un= compounded Colorsare unchangeable,and that Compounded ones are changeable only by refolving them into the colors, of which they are compounded ; and that all the Changes, which can be wrought in Colours, are effected only by vartoufly mixing or parting them : But he grants them on condition that I will explicate Colors by the two fides of a fplit pulie, and fo make but two Jpeciet of them, accounting all other Colors in the world to be but various degrees and di- Iutings of thofe two. And he further grants, that Wbitenefle \s. produced by the Convention of all Colors; but then I mult al» low it to be not only by Mixture ofthofe Colors, but by a far- ther Uniting of the parts of the Ray fuppofed to be formerly fplit. If I would proceed to examine thefe his Explications , I think it would be no difficult matter to fhew, that they are not only infufficientfbut in fome refpectsto me ("at leaft) un*inteM. gible> For, though it be eafie to conceive, how Motion may be dilated and fpread, or how parallel motions may become di- verging} yet Iunderftand not, by what artifice any Linear motion can by a refracting fuperficies be infinitely dilated and rarefied, fo as to become Superficial: Or, if that be fuppofed, yet I underftand as little, why it mould be fplit at (6 fmall an angle only, and not rather fpread and difpe«-fed through the whole angle of Refraction. And further, though I can eafily imagine, how Unlike motions may crofs one another ; yet I cannot well conceive,how|they mould coalefce into one uniform motion, and then part again, and recover their former Un- likenefs 5 notwithstanding that I conjecture the ways,by which the Animadtterfor may endeavour to explain it. So that the Di« rect, uniform and undifturbed Pulfes mould be fplit and di- fturbed by Refraction g and yet the Oblique and difturbed Pulfes perfift without (plitting or further difturbance by fol- lowing Refractions, is (to me) as unintelligible. And there is as ( S°9« ) as great a difficulty in the Number of Colours v as you will fee hereafter. But whatever be the advantages „ _ . . .., \ . _. _r -u- r, A a ;!■ o-nttttamt tuetfTart, u limit #r or disadvantages of this Hypothecs, (Mftilin mf Do a rm 7 ,/*„„, Hype . I hope I may be excuied from ta- thefts. king k up, fince I do not think it needful to explicate my Doctrine by any Hypetb/fis at all. For if Light be confider'd abftra&edly without refpettto any Hy- pothecs, I can as eafily conceive, that the feveral parts or a Ali- ning body may emit rays of differing colours and other quali* ties, of all which Light is conftituted, as that the feveral parts of a falfe or uneven firing, or of uneavenly agitated water in a Brook or Cataract, or the feveral Pipes of an Organ infpi- redall at once, orallthe variety of Sounding bodies in the world together, mould produce founds of feveral Tones, and propagate them through the Air confufedly intermixt. And, if there were any natural bodies that could refleB founds of one tone, and ftifle or tran/mit thole of another ; ^then, as the Echo of a confuted Aggregatof all Tones would be that particular Tone, which the Echoing body is difpofed to reflect ; fo,fiuce (even by the Artimadverfor's conceflions) there are bodies apt to reflect rays of one colour, and ftifle or tranfmit thofe of ano- ther j I can as eafily conceive, that thofe bodies, when il!um> nated by a mixture of all colours, muft appear of that colour only which they reflect. But when the Objector would infinuate a difficulty in thefe things, by alluding to Sounds in the ft ring of a Aiufical inftru- ment before percuffion,or in the Air of an Organ Bellowes be- fore its arrival at the Pipes ; I muft confefs, Lunderftand it as little, as if one had fpoken of Light in a piece of Wood before it be fet on fire, or in the oyl of a Lamp before it afcend up the match to feed the flame. You fee therefore, how much ...„.,. ,. , . . . • • l. rj<..*u~ur ~( • u j 1- Tht difficulties of the Animadver« it is befides the buiiueis in hand, f ors di £ H . rj - e mi fi n g.j /»„ H r tO diipUte about HypOtbefis. For pothefes, and cmjtdtr'd more £tn,* which reafon I (hall now in the ruUy ' laft place,prcceed to abftraft the difficulties in the Animadverfor's difcomfe, and,wichout having regard to any Eypotkefis, confider them in general ternv. And they may be reduced to thefe 3 ^ueres : Hill j,Whe- ( S°92 ) i. Whether the unequal Refra&ions, made without refpeflr to any inequality of incidence, be caufed by the different Re- frangibility of fevera! Rays ; or by the fplitting, breaking or diffipating the fame Ray into diverging parts > 2. Whether there be more than two forts of Colours ? 3. Whether Whitenefs be a mixture of all Colours ? „ , L .,. The Firfl of thefe Quaret you 3. That the R±y it not fplit, cr »itj c s \ J J ^~ • . «' . .ihcrmjdiiat! may £<"* already determmd by an Experiment in my former Let- ter; the defigu of which wastofhew, That the length of the culourd Image proceeded not from any unevennefs in the Glafs, or any other contingent Irregularity in the Refractions. Amongft other Irregularities I kuownot, what is more cbvis ous to fufpe<£t, than a fortuitous dilating and Spreading of Light after foroe fuch manner, as Des-Cartes hath defenbedin his iE.hereal Refractions for explicating the Tayle of a Comer, or as the Animadverfor now fuppofes to be effected by the .Splitting and Rarifyingof his ./Ethereal [pulfes. And to pi c vent the fufpicion of any fuch Irregularities, I told you, that I refracted the Light contrary ways with two Prifmes fuccef- fivdy,to deftroy thereby the Regular effeds of the firfl Prifine by the (eeond, and to difcover the Irregular effeels by augment* ing them with iterated refractions. Now, amongftother Ir* regularities, if the firfl Prifme had fpread and difiipated every ray into an indefinit number of diverging parts, the fecond mould in like manner have fpread aad difiipated every one of thofe parts into a further indefinite Dumber, whereby the I- mage would have been ftill more dilated, contrary to the e- venr< And this ought to have hapned, becaufe thofe Linear diverging parts depend not on one another for the manner of their Refraction, but are every one of them as truly and com- pleatly Rays as \hs whole was before its Ioctdencej as may ap- pear by intercepting them feverally. The reafonablenefs of this proceeding will perhaps better appear by acquainting you with this further circumftance* I fometimes placed the fecond Prifme in a pofition Tranfverfe to thefirft, on defign to try, if it would make the long Image be- come four-fquare by refra&ions croffing thofe that had drawn the round Image into a long one* For, if amongft other Ir- regularities the Refraftien of the jSV/? Prifme, did by Splitting dilate C 5093 ) dilati a Linear ray into a Superficial, the Crofs. refractions of that fecond Prifme ought by further fplitting to dilate and draw that Superficial ray into a Pyramidal folid. Eur, upon tryal, I found it otherwife ; the Image being as regularly Ob- long as before, and inclin'd to both the Prifmes at an angle of 45, degrees. I tryed alfo all other Poficions of the fecond Prifme, by tur- ning the Ends about its middle part; and in no cafe couid ofe ferve any fuch Irregularity. The Image was ever alike incli- ned to 'both Prifmes, its Breadth anfwering to the Suns Dia- meter, and its length being greater or lefs accordingly as the Refractions more or lefs agreed , or contradicted one ano- ther. And by thefe Obfervations, fince the Breadth of the Image was not augmented by theCrofs refraction ofthe/^Wprifme, that refraction muft have been perform'd without any fplitting or dilating of the ray j and therefore at leaft the Light inci- dent on that Prifme muft be granted an Aggregatof Rays ««- equally refrangible in my fenfe. And fince the Image was e- qually inclin'd to both Prifmes, and confequentiy the Refra* Ctions afike in both, it argueSjthat they were perform'd accor- ding to fome Conftam Law without any irregularity. To determine the JectndQux- re,the Animadverfor referrs to an LStf * m """ '**"*"' ° A&i " Experiment made with two Wedge-like boxes , recited in the Micrography of the Ingenious Mr.tfo^Obferv. 10, pag. 73. the de'fign of which was to produce all Colours out of a mixture of two. But there is, I conceive, a double defect in this infiauce. For, it appears not, that by this Experiment all colours can be produced out of two ; and, if they could, yet the Inference would not fol- low* That*// Colours Cannot by that Experiment be produced out of two, will appear by confidering, that the Tincture of Aloet, which afforded one of thofe Colours, was not all over ofone uniform colour, but appear'd yellow near the edge of the Box,and red at other places where it was thicker; affording all variety of colours from zfale yellow to a deep red orScarlet, according to the various thicknefs of the liquor, And f 3 the Lll|ll 2 (eluciou ( S«94 ) folution of Copper, which afforded tire other colour, was of various Blews and ladigo's. So that inftead of tmo colours, here is a great variety made ufe of for the production of all o- thers. Thus, for inflame, to produce all forts of Greens ,the fe- veral degrees of Yellow and pale Blew muft be mixed 5 but to compound Purples, the Scarlet and akep Blew are to be the In- gredients Now, if the Animadverfor contend, that all the Reds and Tel- lows of the one Liquor, or tf/m? and Indigo's of the other, are only various degrees and dilutings of the lame Colour, and not divers colours, that is a Begging of the Queftion ; And I Jhould as fooo grant, that the twoTbirdsot Sixths in Mufick are but feveral degrees of the fame found, and not divers founds* Certainly it is much better to believe our Senfes, in- forming us, that Red aud yellon>~are divers colours, and to make it a Philofophical Quare, Why the fame Liquor doth, accor- ding to its various thicknefs, appear of thole divers colours, than to fuppofe them to be the fame colour becaufe exhibited by the fame liquor ? For., if that were a fufficient reafon, then Blew and Yellow muft alfo be the fame colour., fince they are both exhibited by the fame Tin<9rure of Nephritic kWood. But that they are divers colours, you will more fully underftand by the reafon, which, in my Judgment, is this ■.* The Tincture of Aloes is qualified to tranfmit moft eajily the rays indued with red, moli difficultly the rays indued with violet, and with intermedin at degrees of facility the rays indued with intertnodiat CoIours.So that where the liquor is very thin, it may fuffice to intercept moft of the violet, and yet tranfmit moft of the other colours j all which together muft compound a middle Colour, that is, a faint yellow. And where it is fo much thicker as alfo to inter- cept moft of the Blew and Green, the remaining Green, Yellow, and Red.'u muft compound an Qrenge. And where the thick- nefs is fo great, that fcarce any rays can pafs through itbe- fides thofe indued with 7(#^,'.rnuft appear of that colour,and that fo much the deeper and obfeurer, by how the liquor is thicker. And the fame may be underftood of the various de- grees of Blew, exhibited by the Solution of Copper, by reafon of its difpofition to iutercept J{ed moft eafily, and trtfifmk a deep Slew qi ind/go-Cokm moft freely, But - C 5095 ) But, foppofing that all Colours might,accoTcling to this ex penment,be produced out of trvo by mixture 5 yet it foI!ow s not, that thole two are the only Original colours, and that fo r a double reafon. Firfi i becaufe thofe two are not themfelye 5 Original colours, but compounded ofothers ; there being no liquor nor any other body in nature, whcfe colour in Day. light is wholly uo-compouuded. And then^ becaufe, though thofe two were Original, and all others might be compounded of them, yet it follows not, that they cannot be otherwife produ- ced. For I faid, that they had a double Origin, the fame Co- lours to fenfe being in fome cafes compounded and in others un compounded; and fnfficiently declar'd in my third and fourth Proportions, and in the Conclnfion., by what Properties the one might be known and diftinguiuYc from the other. Bur, becaufe I fufpectby fome Circumftances, that the Dijlinftion might not be rightly apprehended, I fhall once more declare it, and further explain it by Examples. That Colour is Primary or Original^ which cannot by any Art be changed, and whofe Rays are net alike refrangible : And that Compounded, which is changeable into other colours, and whofe Rays are not alike refrangible, For injlartcc, to know, whether the colour of any Green object be compouu* ded or not, view it through a Prime, and if it appear confufed^ and the edges tinged within?, Telloro, or any variety of o- ther colours, then is that Green compounded of fuch colours as at its edges emerge cut of it : But if it appear dtftinfi, and well defin'd, and entirely Green to the very edges, without any other colours emerging, it is of an Original and un-coin- pounded Green. In like manner, ifa refracted beam cf lighr 5 being caft on a white wall, exhibit a Green colour, to know whether that be compounded, refract the beam with au in- terpofedPrifme* and if you find any Diffornnty in the refra- ctions, and the Greenbt transform' d into Blew, Telioivprany variety, of other colours, you may conclude, 'that it was com- pounded of thofe which emerge: But if the Refractions be uniform, and the Green perfift without any change of colour, then is it Original and un compounded^ Ai-idthe reafon why I call it fo, is, becaufe a Green indued with fuch properties can- not be produced by any mixing of other colours, Now C S&97 ) UXovr^ltwo Green Otye&s may to the naked eye appear oi the fame colour, and yet one of them through a prifme feem confu/ed and variegated with other colours at the edges, and the other dijlinU and entirely Green ; or, if there may be two Beams of Light, which falling on a white wall do to the na- ked eye exhibit the fame Green colour, and yet one of them, when tranfmitted through a Prifme, be uniformly and regit-. larly refra&ed,and retain iacolour unchanged, and the other be irregularly refracted and,to divaricate into a multitude of other colours ; I fuppofe, thefe tyro greens will in both cafes be granted of a different Origin and conftitution. And if by mixing colours, a green cannot be compounded with the pros pertiesof the "Unchangeable Green, I think, I may Call tbatm Vn-eompounded colour, especially fince its rays are alike refran- gible,and uniform in all refpects. The fame rule is to be obferv'd in examining, whether }\ed 3 Orenge, Yellorv^Uro^ or any other colour be compounded or not. And, by the way, fince all White objects through the Prifme appear confus'd and terminated with colours, Whitenefs inuft, according to this diftin&ion, be ever compounded, and that the moft of all colours, becaufe it is the moft confus'd and changed by Refracl:ioDS« From hence I may take occafion to communicate a way for the"improvement ofMicrofcepes by Refraction. The way \%, by illuminating the Objcd in a darkncd room with Light of any convenient colour not too much compounded: for by that means the Microfcope will with diftin<3:ue(s bear a deeper Charge and larger Aperture, efpecially if its conftrudfcion be fuch, as I may hereafter de/cribe j for, the advantage in Or- dinary Microfcopes will not be fofenfible. _. . There remains now the third io. That Whit me Is a a mixture of _. _ . /-j >j i • t . *u colour;. Qusere to be conhder d, which is, Whether Wbitentft be an Uniform Colour, or a diffimilar Mixture of all colours } The Experiment which I brought to decide it, the Animadverfir thinks may be otherwise explain'd, and fo concludes nothing. Buthe might eafily have fatisfied himfelf by trying, what would be the re« fult of a Mixture of all colours. And that very Experiment might have fatisfied him, if he had pleafed to examine it by the C ^96 y the various circumftances. One circumftance I there decla- red, of which I fee no notice taken; anditis, That if any co* Jour at the Lens be intercepted, the Whitenefj willbe changed into the other colours .' If all the colours but red be intercep. ted, that Red alone in the concourfe or croffing of the Rays will not conftitute Whitenefs, but continues as much Red as before* and fo of the other colours. So that the bufinefs is not only to fhew, how rays, which before the coneourfe exht- bit colours, do in the concourfe exhibit White 5 but to fhcvv, How in the fame place, where the feveral forts of rays apart exhibit feveral colours,a Confufion of all together make White, For inftance, if red alone be firfttranfmitted to the paper at the place of coneourfe, and then the other colours be let fall on that Red, the Queftion will be, Whether tb.y cpnvert it in-* t> White, by mixing with it only, as Blew falling on Yellow light is fuppos'd to compound Green 5 or. Whether there be fume further change wrought in the colours by their mutual acting on one another, until}, like contrary Peripatetic quz\b ties, they become affimilated. And he that fhall explicate this, laft Cafe mechanical!}^ muft conquer a double impoffibil fy. He mufl/frT? fliew, that many unlike motions in a Fluid can by clafhingfoaft on one another, and change each other, as to become one Uniform motion ; andt&eu, that an Uniform mo- tion can of it feif, without any new unequal impreffions, de- part into a great variety of motions regularly un-equal. And after this he muft further tell me, Why all Objects appear not of the fame colour, that is, why their colours in the Air, where the rays that convey them every way are confufedly mixt, do not affimilate one another and become Uniform be- fore they arrive at the Spectators eye.'' But if there be yet any doubting/tis better to put the Event on further Circumftances of the Experiment^ than to 2cquiefce in the poffibility of any Hypothetical Explication. As, for in- ftance, by trying,What will be the apparition of thefe colours in avery quick ConfecutioD of one another. And this may bs eafily perform'd by the rapid gyration of a Wheel with [many Spoaks or coggs in its perimeter, whofe Interfaces and thick* nefles maybe equal and of fuch a largenefs, that, if the Wheel be inter poled between the Priftne and the white coneourfe ( $098 ) of the colours, one half of the Colours may be intercepted by a fpoake or cogg, and the other half pafs through an inter* ftice. The Wheel being in this pofture, you may firft turn it flowly about, to fee all the colours fall fucceffively on the fame place of the paper, held at their aforefaid concourfe $ and if you then accelerate its gyration, until the Confecution of thofe colours be fo quick, that you cannot diftinguifli them feverally, the refulting colour will be a Whitenefs per- fectly like that, which an mn-refracted beam of Light exhibits, when in like manner fucceffively interrupted by the fpoaks or coggs of that circulating Wheel* And that this Whitenefs is produced by a fucceffive Intermixture of the Colours, with- out their bdng affimilated , or reduced to any Unifor- mity, is certainly beyond all doubr, unlefs things that exift not at the lame time may notwithstanding ad on one a» nother. There are yet other Circumftances, by which the Truth might have been decidedj as. by viewing the White concourfe of the Colours through another Prifme plac'd clofe to the eye, by whole Refraction that whitenefs may appear again tranf- form'd into Colours : And then, to examine their Origin, if an Affiftant intercept any of the colours at the Lens before their arrival at the Whitenefs, the fame colours will vanifh from a- mongft thofe, into which that Whitenefs is converted by the fecond Prifme, Now, if the rays which difappear be the fame with thofe that are intercepted, then it muft be acknowled* ged, that the fecond Prifme makes no new colours in any rays, which were not in them before their concourfe at the paper. Which is a plain indication, that the raysoffeveral colours re- main difticcl: from one another in the Whitenefs, and that from their previous difpofitions are deriv'd the Colours of the fecond Prifme. And, by the way, what is faid of their Colors may be applied to their Refrangibility. The aforefaid Wheel may be alfohere made ufe of 5 and, if its gyration be neither too quick nor two flow, the fuc- cefsionof the colours may be difcern'd through the Prifme, whilft to the naked eye of a Byftander they exhibit white, fiefs. There is fomethiog ftiil remaining to be faid of this Expert. raent ( S°5>9 ) raent. But this, I conceive, is enough to enforce it, and (o to de-J cide the controverfy. How-ever, I (hall now proceed to (hew fome other, ways of producing Whitenefs by mixtures , iince I pcrfwade my felf, that this Affertion above the reft appears Paradoxical, and is with raoft difficulty admitted. And becaufe the Animadvcrfor -iefires an inftance of it in Bodies of divers colours, I (hall begin with that, Bucinorder thereto itmuft be confider'd, that fucli colour d Bodies reflect but fome part of the Light incident on them ; as is cvidenc by the 1 3 Propofition .- And therefore the Light reflected from an t\g- gregat of them will be much weakned by the lofs of many ray?: Whence a perfect and intenfe Whitenefs is not to be expected, but rather a Colour between thofe of Light and Shadow, or fuch a Gray or Dirty colour as may bemade by mixing White and Black together. And that fuch a Colour will refult, may be collected from the colour of Dufl found in every corner of an boufe, which hath been obferv'd to confift of many coloured particles. There may be alfo produced the like Dirty colour by mixing feveral Painters colours together. And the fame may be effected by Painting a Top (fuch as Boys play with) of divers colours. For, when it is made to circulate by whipping it, it will appear of fuch a dirty co^ lour. Now, the Compounding of thefe colours is proper to my pur- pofe, becaufe they differ not from Whitenefs in the Species of co- lour, but only in degree of Luminoufnefs ; which (did not the An- imadverfer concede it) I might thus evince. A beam of the Suns Light being tranfmitted into a darkned room, if you illuminate a iheet of White Paper by that Light, reflected from a body of a- ny colour, the paper wilt alwaysappear of the colour of that bo- dy, by whofe reflected light it is illuminated. If it be a red bo- dy, the paper will be red, if a green body, it will be green; and fo of the other colours. The reafon is, that the fibers or threds, of which the paper confifts, are all transparent and fpecular ; and fuch fubitances are known to reflect colours without changing them. To know therefore, to what Species of colour a Grey belongs, place a- ny Gray body(fuppofe a Mixture of Painters colours t )\n the laid Light, and the paper, being illuminated by its reflexion, (hall appear White. And the fame thing will happen, if it be illuminated by reflexion from a to^fubitance, Thefe therefore are all of one Species ; but yet they feem diftin- guifht not only by degrees of Luminoufnefs, but alfo by fome other Inequalities, whereby they become more harfli or pleafant. And the diftinction feems to be, that Greys and perhaps Blackj are made by an uneven defect of Light, confuting as it were of many little veins or ftrcams, which differ either in Luminoufnefs or in the Unequal di- M m m m ra ftributiont ( 5i°» ) ftributionofdiverfly colour'd rays-, fucb a* ought to be caus'j by Reflexion from a Mixture of white and black, or of diverfly co. lour'd corpufcles. But when fuch imperfe&ly mixt Light is by afe- cond Reflexion from the paper more evenly and uniformly "blended, it becomes more pleafant, and exhibits z faint or fliadowM White- nefs. And that fuch little irregularities as thefe may caufe thefe dif- fere«ces, is not improbable, if weconfider, how much variety may becaufedin Sounds of the fame tone by irregular and uneven jar- tings. And befides , thefe differences are fo little, that I have fome- times doubted, whether they be any at all, when I have confider'd that a Black and White Body being plac'd together, the one in a ftrong light, and the other in a very faint light, fo proporti- on'dthat they might appear equally luminous h it has been dif- ficulty difUnguifh thera, when view'd at diftance.unlefs when the Black feem'd more blewifli j and the White body in a light {till fainter, hath, in companion of the Black body, it felf appcar'd This leads me to another way of Compounding JVhitenefs ; which is That if four or five Bodies of the more eminent colours; or a Faper painted all over, in feveral parts of it, with thofe feveral colours in a due proportion, be placed in the faid Beam of Light} the Lieht reflc&ed from thofe Colours to another White paper, held at a convenient diftance, fhallmake that paper appear White; If it be held too near the Colours, its parts will fcem of thofe colours that are neareft them ; but by removing it further, that all its parts mav be equally illuminated by.all the colours, they will be more and mo« diluted, until they become perfectly White. And you may Z Ser ob?erve,thatif any of the colours be intercepted, the Paper wU no longer appear White, but of the other ^ colours which i are not Tnt rceited Now, that this Htemfs « a Mixture of the federally S«n„r»Travs fallinc confufedly OB the paper, I fee no reafon to 2 W becaufe if the Light became Uniform and Similar before ttl confu ed? on the papef, it muft much more be Uniform when ataUawr diftance it falls an the Speftitor. eye and fo ; the ray ,, ac a Brc4v<.j- u i c0 i ours , wou d in no qualities differ from *S,t XlrftoSfi*. to. colourto ***. tor, contrary to what he fees. Not much unlike this Inftanceit is,That,if a polifht piece of Meta! be ft pUced that the colours appear in it as ma Looking- gl.ft, ™d then the Metal be made rough, that by a confusd reflcv • link Vooarent colours may be blended together, they Iha 1 SppearVSb" their uta&rc caufe the Metall to look White. Bat C $101 ) But further to enforce this Experimtnt i if, inftead of the Paper, 1 any White Froth, confifting of fmall bubles, be illuminated by re* flexion from the aforefaid Colours, it /hall to the naked eye feem White, and yet through a good Microfcope the feveral Colours will appear diftind onthebublcs, as iffecn by reflexion from fo many fpherical Surfaces. With my naked eye, being very near, I haveaf. fo difcern'd the feveral colours on each buble $ and yet at a greater diftance, where I could not diftinguifli them apart, the Froth hath appear'd entirely White. And at the fame diftance, when I look'd intently, I have feen the colours diftindly on each buble ; and yet, by draining my eyes as if I would ldok at fomething far off beyond them, thereby to render the Vifion confus'd, the Froth has 3ppear'd without any other colour than Whitenefs. And what is here fail of Froths,may eafily be undcrftood of the Paper or Metal in the fore- going Experiments. For, their parts are fpecular bodies, like thefe Bubles •• And perhaps with an excellent Microfcope the Colours may be alfo feen intertwixedly reflected from them. In proportioning the feveral ly Colour'd bodies to produce thefe efreits,therc may befome nicenefs} and it will be more convenient, tomakeufe of the colours of the Prifme, call: on a Wall, by whofe reflexion the Paper, Metal, Froth, and other White fubftances may. be illuminated. Andlufually made my Tryals this way, becaufc I could better exclude any fcattering Light from mixing with the colours to dilate them. fr To this way of Compounding Whitenefs may bereferr'd thato. ther, by Mixing light after it hath been injected through tranfpa- rently colour'd fubftances. Forinft/tnce, if no Light be admitted in- to a room but only through Colour'd glafs, whofe feveral parts are of feveral colours in a pretty equal proportion; all White things in the room (hall appear White, if they be not held too near the Glafs. And yet this light, with which they are illuminated, can- not poflibiy be uniform, becaufe, if the Rays, which at their entrance are of divers colours, do in their progrefs through the room fuffer any alteration to be reduced to an Uniformity ; the Glafs would not In the remote!! parts of the room appear of the very fame colour, which it doth when the Spectators eye is very near it : Nor would tke rays, when tranfmitted into another dark room through a little hole in an oppolite door or partition-wall, project on a Paper the Species or reprefentation of the glafs in its proper colours, And, by the by, this fcems a very fit and cogent Inftance of fome other parts of my Theory, and particularly of the 13 Proportion. For, in this room all natural Bodies whatever appear in their proper co- lours. And all the Phxmmena of colours in nature, made either fay Refraction or without it, are here the fame as in the Open Air. Now, the Light i.n this room being fuch a DiflimiUr njixtu.e, s< M m m m ra z i { 51*2 ) * have defcrib'd in my Theory, the Caufes of al! thefe Hw.omenA mull be the fame that 1 have there aflign'd. And I fee no reafon to fulped, that the fame Phenomena i\\oxx\ terminates > which bounds he conjectures to extend thenafelves, beyond Saturn, to thofe Fixt Stars that are of the nearer rank to S*-- turns Orbe.. Concerning the Bodies lodged in thefe Planets, he thinks it con- fonantto the Power and Wifdom of the Great Creator, that there fhould be fuch a variety of them,, as 1 0. ftock each of ih<; (aid Planet*, wlr.lt